It is known that microorganisms can grow in aircraft fuel tanks thereby causing problems such as filter blockage and corrosion damage. Biocidal fuel additives have been used to control the growth of such microorganisms. 2-Methoxyethanol is a common fuel additive primarily used to prevent icing within the fuel system but also recognized as having biocidal properties as high usage levels of about 5000 parts per million (ppm). BIOBOR JF.RTM. fuel fungicide (a trademark of U.S. Borox & Chemical Corp.) is reported to be more effective than 2-methoxyethanol (See Hill, E. C., Journal of the Institute of Petroleum, 56 (549), 138-146 (1970) and U.S. Pat. No. 3,877,890). While BIOBOR JF.RTM. fuel fungicide (a combination of organoborates) is particularly effective against fungi, e.g., Cladosporium resinae, other microorganisms found in fuel systems such as bacteria, e.g., Pseudomonas sp., are not as easily controlled by the combination of organoborates.